The present invention is directed to a wavefront sensor, such as a sensor for wavefront aberrations in the eye, and more particularly to such a sensor which avoids corneal reflection by illuminating the retina along a light path off of the optical axis of the eye. The present invention is further directed to a method of sensing a wavefront using such off-axis illumination.
It is known in the art to detect wavefront aberrations in the human eye for such purposes as intraocular surgery and contact lens fabrication. Such detection is disclosed, e.g., in Liang et al, "Objective measurement of wave aberrations of the human eye with the user of a Hartmann-Shack wave-front sensor," Journal of the Optical Society of America, Vol. 11, No. 7, July, 1994, pp. 1-9. A beam of light from a laser diode or other light source is directed toward the pupil and is incident on the retina. Because the retina is highly absorbing, a beam on the order of four orders of magnitude dimmer than the original beam is reflected by the retina and emerges from the pupil. Typically, the incoming and emergent light follow a common optical path; the incoming light is brought into the common optical path with a beamsplitter.
The emergent beam is applied to a Hartmann-Shack detector to detect the aberrations. Such a detector includes an array of lenslets which break up the light into an array of spots and focus the spots onto a charge-coupled detector or other two-dimensional light detector. Each spot is located to determine its displacement from the position which it would occupy in the absence of wavefront aberrations, and the displacements of the spots allow reconstruction of the wavefront and thus detection of the aberrations.
Improvements to the technique of Liang et al are taught in J. Liang and D. R. Williams, "Aberrations and retinal image quality of the normal human eye," Journal of the Optical Society of America, Vol. 4, No. 11, November, 1997, pp. 2873-2883 and in U.S. Pat. No. 5,777,719 to Williams et al. Williams et al teaches techniques for detecting aberrations and for using the aberrations thus detected for eye surgery and the fabrication of intraocular and contact lenses. Moreover, the techniques of those references, unlike that of the Liang et al 1994 article, lend themselves to automation.
The techniques described above involve illuminating the eye along the eye's optical axis. As a consequence, the light reflected from the retina is mixed with stray reflections which can disrupt measurements. More specifically, the stray reflections show up as spurious bright spots amid the array of spots formed in the Hartmann-Shack sensor.
Such stray reflections have several sources in wavefront sensors. Of particular concern are the reflections from the optical elements between the retina and the beamsplitter. Such elements typically include the optics of the eye and a pair of lenses between the beamsplitter and the eye. Back reflections from surfaces other than the retina are weak relative to the illuminating beam but are bright relative to the weak signal reflected from the retina.
In the eye's optics, the only surface whose back reflection is bright enough to be problematic is the first (outer) surface of the cornea. That reflection is comparable in energy to the reflection from the retina and can therefore be a considerable nuisance for wavefront sensing, particularly if the centroids of the spots in the detector are to be computed automatically.
One known way to remove the corneal reflection, taught in Liang and Williams and in Williams et al, uses a polarizing beamsplitter to remove reflected light from all of the surfaces between the beamsplitter and the retina. Because those surfaces retain the linear polarization of the light incident thereon, both lens reflections and the corneal reflection are eliminated. However, much of the light reflected from the retina is also lost. Only depolarized light reflected from the retina, which accounts for only about thirty percent of the total light reflected from the retina, is available to detect the wavefront aberration. Moreover, the depolarized light contains considerable spatial noise. Still another problem is the intensity nonuniformity introduced into the array of spots by the birefringence of the eye's optics, chiefly the cornea.
Another known way to remove reflections from all optics between the beamsplitter and the eye while increasing the signal from the retina involves the use of a polarizing beamsplitter in combination with a quarter-wavelength (.lambda./4) plate just in front of the eye. German published patent application No. DE 42 22 395 A1 teaches that technique. That technique allows a much greater part of the light reflected from the retina to reach the detector, thereby improving spot quality, while removing the variation in spot brightness caused by the birefringence of the eye. It also removes back reflection from the lenses. However, the corneal reflection is not removed and is thus just as troublesome as it would be in the absence of polarizing optics.
Another problem with the two techniques just described is the cost of the polarizing beamsplitter and of the .lambda./4 plate. In cost-sensitive commercial settings, it would be desirable to eliminate that cost.